In light of the problem of ozone layer depletion in recent years, restrictions on CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) used as refrigerants in conventional refrigerating machines have become more stringent, and HFCs (hydrofluorocarbons) are coming into use as substitute refrigerants. However, HFC refrigerants are also associated with problems such as increased contribution to global warming, and the use of natural refrigerants as substitutes for such fluorocarbon refrigerants is currently being researched. Among such natural refrigerants, carbon dioxide refrigerants are known to be harmless to the environment and highly safe, while also having advantages such as compatibility with oils and mechanical materials and being readily available, and they have also been used as refrigerants for refrigerating machines in the past. Research has also recently begun on their use as refrigerants for automobile air conditioners that employ open type-compressors or closed type-electrical compressors.
Patent document 1 listed below has disclosed a refrigerating machine oil comprising an esteric base oil, as a refrigerating machine oil used with a carbon dioxide refrigerant.    [Patent document 1] Japanese Unexamined Patent Publication No. 2000-104084